A user that uses "info gcc" to look for details about the preprocessor
doesn't find them. Example: [Bug c/29994]. The user did "info gcc",
was looking for a section that explains all the predefined preprocessor
macros, but didn't find it. The reason was that they are in a separate
manual.
Nowadays the preprocessor is rarely used on its own. To the user, it
appears as part of the compiler.
Suggestion: Integrate the parts of cpp.info that don't apply to other
C/C++ implementations (such as "Predefined Macros" and "Invocation", and
possibly others) into the gcc.info manual.